Current-mode logic (CML) is a differential digital logic family that is used for high speed data transmission. A CML device typically employs a differential amplifier input stage to perform a logic operation, and can include a follower stage to level shift the outputs to a voltage level that is compatible with the input voltage level, and to drive the differential outputs. Input voltage levels in a CML device are typically low, and the differential amplifier input stage is typically not operated in full swing between two power supplies, resulting in fast switching of logic states. The differential amplifier input stage is driven by a constant current source (e.g., an emitter- or source-resistance) that provides a negative feedback and results in a high input impedance. Both legs of the differential amplifier input stage typically include large load resistors that provide the differential outputs. In operation, one leg of the differential amplifier input stage is typically in cut-off, while the other leg is typically operated in the active linear region, resulting in substantially all of the current running in the active leg.
The bandwidth of the differential amplifier input stage of a CML device is fundamentally limited by the RC time constant associated with the load resistors in combination with the input capacitance of the follower stage or of the next logic device. As such, the data rate that can be achieved by a CML device is limited. As the data rates of various communication technologies increase, there is an increasing need for a CML device that has a higher bandwidth, and that can thus support a higher data rate. Also, the output of the differential amplifier input stage of the CML device is typically provided at different voltage ranges, based upon the configuration of the CML device, and so one or more follower stages may be needed to couple the CML device to another type of device. Thus there is also a need for CML devices that provide a fixed output voltage.
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